Hi @Hakim,
Is your nextcloud already installed or not?
if you just have extracted the archive and move it into /var/www/, make sure www-data ( apache user have the permission on your nextcloud folder. Maybe try these lines : sudo chown -R www-data:www-data /var/www/nextcloud/ sudo chmod -R 750 /var/www/nextcloud/
I assume, what Mageunic wrote above will do the trick. Even that the first command should suffice afaik. That step is indeed missing on the example installation.
Anyways, I can really recommend, to read through the full Installation on Linux Section, and eventually write down some notes and therefore build your own instructions. It’s indeed more to read, but it also is more information and steps towards a properly configured setup.
@Michael_Beck which type of installation have you? ( snap? docker? manual installation?)
have you set a specific path for data directory of nextcloud ?